With the use of nondispersive gas concentration measuring devices, i.e., gas analyzers which operate on optical principles, and which contain neither gratings nor prisms, the measured value is customarily obtained from the comparison of two light fluxes, i.e., one light flux or beam which is strongly influenced by the measured object and one light flux or beam which is influenced very little, or not at all, by the substance being measured.
Processes and devices are known for this type of concentration measurement wherein the light beams simultaneously pass through optical paths which are geometrically separated from each other, or, wherein both light beams pass through the same optical path in succession.
In the second process mentioned above, it is possible to use two filter cells filled with different gases or dissolved substances or two interference filters, of which the center wavelengths differ from each other, wherein the filter cells or the interference filters are periodically and serially inserted in the beam path. The filter functions are thereby so selected that only one of the two light beams at a time, in sequence, can be influenced by the substance being measured. Measuring devices in accordance with the second process mentioned above are when using gases or dissolved substances as a filter, (this being referred to as negative selective modulation) gas filter correlation photometers, and when using interference filters (positive selective modulation) interference filter correlation photometers. Either correlation or bifrequency photometers can be used in conjunction with the same electronic analysis device.
A correlation photometer is shown in the Review of Scientific Instruments, Vol 49, page 1520 (1978), in which the ratio (I.sub.M -I.sub.R)/I.sub.R is used as the measure of the concentration of the measured substance in the optical beam path, wherein I.sub.M and I.sub.R indicate the intensities of the measured and reference signals, respectively. The differential signal (I.sub.M -I.sub.R) is, thus, obtained by phase-sensitive modulation of the detector signals having the frequency .omega., wherein .omega. is the modulation cycle frequency.
To determine I.sub.R, the light intensity during the reference signal phase is also mechanically modulated with the frequency .omega.', which is far higher than .omega. and the detector signal is modulated with the frequency .omega.' in respect to the phase. Thereafter the ratio is formed.